The present invention relates to an image forming apparatus used for a graphic type non-impact printer, and particularly to an image forming apparatus in which toner is fed to an electrostatic latent image bearing medium.
In general, an image forming apparatus in a graphic type non-impact printer is operated by a process comprising the steps of electric charge, exposure, development, transfer and fixation.
When a toner cartridge is set in the image forming apparatus, toner stored therein is dropped onto an agitating shaft and guide plate.
At this time, when a main motor is energized, a supply roller, a developing roller and a photosensitive drum which are engaged to a gear on the shaft of the main motor are rotated.
Toner is fed toward the supply roller from the agitating shaft through holes in a guide plate, and is then fed to the developing roller by the rotating supply roller. Toner sticking to the outer surface of the developing roller is turned into a thin layer by a developing blade, and is then fed toward the photosensitive drum. At this time, the toner is negatively charged. Meanwhile, the outer surface of the photosensitive drum is uniformly and negatively charged by a charger, and is then exposed to light by a printing head in accordance with image signals. Then, negative charge is released from a part which is exposed to the light. Accordingly, an electrostatic latent image corresponding to an image to be printed is formed by the part which is exposed to the light and the part which is not exposed to the light.
When the photosensitive drum is rotated so that the section on which the latent image is formed comes to a position where it faces the developing roller, it attracts the negatively charged toner so as to effect development. The image on the outer surface of the photosensitive drum is transferred from the latter to a sheet by the charger when the sheet passes between the photosensitive drum and the charger. The image obtained by the transfer of the toner image is thereafter is fixed by a fixing unit.
Further, the toner remaining on the outer surface of the photosensitive drum after the transfer is scraped off by a cleaning blade so as to stand by for the next printing. The toner scraped off is fed toward the agitating shaft by a screw shaft so as to be reused.
It is noted that the toner which has been retained in a retaining part is fluidized circulately by the supply roller and the developing roller which are rotated.
However, in the above-mentioned apparatus, with the repetitions of printing, paper dust from sheets sticks to the outer surface of the photosensitive drum, and is then mixed into the remaining toner after being scraped off by the cleaning blade. Since the remaining toner is reused, the paper dust mixed therein is also sent to the agitating shaft together with the toner. Further, when the toner on the developing roller is turned into a thin layer by the developing blade, the paper dust is caught between the developing roller and the developing blade, and accordingly, is sometimes held therebetween since it cannot pass. In such a case, the toner is scraped off from the developing roller by the paper dust held therebetween. Thus, there has been raised such a problem that the toner cannot be uniformly fed onto the photosensitive drum. That is, in this case, when an image is transferred onto a sheet, the part of the sheet onto which no toner is fed, is left as a white line. Further, such a white line is sustained until the paper dust is removed naturally.
Accordingly, one object of the present invention is to provide an image forming apparatus in which the rotation of a developing roller is periodically reversed so as to remove paper dust held between a developing roller and a cleaning blade.